The present invention relates to electronically testing electrochemical cells and batteries. More specifically, it relates to method and apparatus for passing a programmably-determined periodic current through an electrochemical cell or battery to facilitate measurement of at least one component of its ac immittance (i.e., either ac impedance or ac admittance) at a specific frequency.
Electrochemical cells and batteries, such as primary cells/batteries, secondary (i.e., storage) cells/batteries, and fuel cells/batteries are important sources of electrical energy. As such, their complex impedance/admittance is of both theoretical and practical interest. Recent U.S. patents issued to Champlin disclose methods and apparatus for. accurately measuring components of complex impedance (U.S. Pat. No. 6,002,238; U.S. Pat. No. 6,172,483) and complex admittance (U.S. Pat. No. 6,262,563) of cells/batteries at a specific frequency. A common feature of these inventions is that they all employ a periodic currentxe2x80x94a current that need not be sinusoidalxe2x80x94to excite the cell/battery undergoing test.
Consider FIG. 1. This figure depicts immittance-measuring apparatus disclosed in the prior art U.S. Pat. Nos. 6,002,238, 6,172,483, and 6,262,563 and shows details of current excitation circuitry disclosed therein. Current exciter 5 comprises a series combination of load resistor 25 and controlled switch (i.e. transistor) 30 connected to cell/battery 10 through current-carrying contacts A and B. A symmetrical timing signal 70 outputted by microprocessor/controller 20 turns controlled switch 30 xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d at the measurement frequency f. Accordingly, a square-wave current xe2x88x92i(t) at frequency f flows through: the cell/battery in the discharging direction as shown. (By convention, cell/battery current is assumed positive in the charging direction.) The peak to peak amplitude and average value of this generated square wave are |VB/RL| amps and xe2x88x92(VB/2RL) amps, respectively, where VB is the cell/battery voltage and RL is the load resistance. Current exciter 5 also outputs a signal voltage RLi(t) 35 for processing by the remaining measurement circuitry. The function and operation of all other elements depicted in FIG. 1 have been fully explained in the referenced Champlin patents and will not be repeated herein.
One problem with this prior art current exciter is that the excitation current is inevitably a discharging current. There is, however, ample theoretical basis for believing that immittance measured with zero net current, or even with a net charging current, is equally important. Furthermore, the amplitude: of the generated square-wave in this prior art circuit is fixed at a value determined by the cell/battery voltage and the resistance of the load resistor. This fixed amplitude may not be large enough to develop sufficient ac voltage across low-impedance cells/batteries for accurate measurement. Or, it may be so large that high-impedance cells/batteries are driven into nonlinearity. All of these objections to the method disclosed in the prior art are surmounted by the inventions disclosed herein.
The programmable current exciter disclosed herein bears some resemblance to the xe2x80x9cflying bridgexe2x80x9d circuit disclosed in FIG. 5 of PCT Application WO 99/18448. However, a careful comparison of the two inventions reveals very significant differences in the objectives, implementation and results achieved.
The present invention comprises an exciter of periodic square-wave current for use in measuring one or more components of complex ac impedance or admittance of a cell or battery. A microcontroller/processor outputs two digital words that define upper and lower current levels. These words are latched and converted to analog voltages by D/A converter circuitry. A timing signal at the measurement frequency, also outputted by the microprocessor/controller, controls a multiplexer arranged to select either analog voltage. The multiplexer output thus toggles between the two programmed analog voltages at the measurement frequency.
By virtue of negative feedback, the toggled multiplexer output voltage equals the voltage developed across a resistance in series with the cell/battery. Two complementary transistors and a dc voltage source are arranged such that a positive multiplexer output directs a programmed current through this resistance in the xe2x80x9cdischargexe2x80x9d direction, and a negative multiplexer output directs a programmed current through it in the xe2x80x9cchargexe2x80x9d direction. Accordingly, the current through the cell/battery is a symmetrical square wave having frequency, amplitude, average value, and average flow direction completely under program control.